1. Field of the Invention
This invention relates to grip apparatus for use with tensile stress testing apparatus providing a relatively light weight, strong grip assembly which enables the rapid and easy removal and replacement of the grip jaw inserts, without disassembly of the grip apparatus from the testing apparatus.
2. Description of the Prior Art
In the prior art, tensile stress testing apparatus has been subject to many disadvantages, especially when such prior art testing apparatus is used for testing high tensile strength materials subject to tensile stresses in the range of several tens of thousands of pounds per square inch. The prior art devices generally are constructed of heavy materials resulting in substantial total weight. In addition, the prior art devices involve complicated structures that are expensive to manufacture. Furthermore, removal and replacement of grip jaws in prior art devices requires substantial disassembly of the grip apparatus and often requires removal of the grip apparatus from the testing device.
Canadian Pat. No. 446,096 describes grip apparatus comprising a pair of rectangular grip blocks, each of which is secured in one of the crossheads of the tensile stress testing apparatus. Each grip block has a V-shaped recess which contains one or more grip inserts. The grip insert is secured in the grip block by an aligning plate at the bottom of the block and by a removable washer at the top of the block. To provide more positive gripping action, the gripping face of each insert slopes inwardly (toward the test piece) toward its upper end and the back surface of each insert is provided with finely cut teeth or cross-cut ridges to provide a more positive contact between the V-shaped recess and the back of the grip insert. That patent does not disclose the grip housing containing the readily removable and adjustable grip bars and grip inserts of the apparatus of this invention.
Soviet Union Pat. No. 800,797 discloses a grip housing having four rectangular grooves arranged in a cross-shaped pattern. Each groove is rectangular in transverse cross-section and secures a rectangular grip insert. Each insert has a verticle face adapted to engage the test piece and a sloping slide surface which engages a sloping surface in the groove of the grip housing. Each pair of opposing grooves in the housing forms a matched pair, each being a mirror image of the other. Each sloping side wall of one pair of grip inserts forms an angle (a) with the longitudinal axis of the housing, and each sloping side wall of each of the other pair of inserts forms a different angle (b) with the longitudinal axis of the housing. The ratio of those inclination angles is equal to the ratio of the transverse strains in the two perpendicular directions and is also equal to the ratio of the Poisson coefficients in the mutually perpendiculr directions. In that device, the end of the test piece secured in the grip jaws must be rectangular in transverse cross-section and must be of sufficient size to abut the surfaces of all of the grip jaws in a substantially close tolerance fit. That patent does not disclose the apparatus of this invention comprising a housing containing a pair of grip bores each containing a grip bar and grip jaws which are easily and readily removable and replaceable without substantial disassembly of the apparatus.
U.S. Pat. No. 2,419,711 describes grip apparatus designed to withstand test loads of up to 10,000 pounds. The apparatus includes a slotted block of steel forming a base member which supports a pair of steel block jaw holders having inclined jaw engaging faces. Each of a pair of grip jaws has a slot in its back surface engaging a pin extending from the inclined surface of the jaw holder. That device is held in assembly by a clamp band and set screw, which is used to apply the initial gripping force to the test piece. That device is a relatively heavy assembly of complicated construction that requires complete disassembly of the apparatus to permit removal or replacement of the grip jaws. In addition, that patent does not disclose the apparatus of this invention comprising a light weight grip assembly capable of sustaining test forces in excess of 30,000 pounds and including a housing having grip bores containing grip bars supporting grip jaws that are easily and readily removable and replaceable without substantial disassembly of the apparatus.
U.S. Pat. No. 2,537,322 describes a grip apparatus having a relatively complicated structure comprising at least a threaded shank, a clamp head, a clamp nut, and a pair of clamp jaws, all of which must be actuated cooperatively to provide the required gripping force. The device shown in that patent weighs approximately one pound but permits tensile stress only to approximately 250 pounds when the parts are made of ordinary steel. When high tensile stresses are required, the structural elements must be made of special high tensile steel and the structural elements must be of larger size. That patent does not describe a structure capable of operating at tensile stresses in excess of 30,000 pounds per square inch, such as that described in this invention. In addition, that patent does not disclose the structure including a grip housing enclosing readily removable grip bars on which are mounted grip jaws that are easily and readily removable and replaceable from the apparatus without substantial disassembly of the apparatus.
U.S. Pat. No. 3,403,549 discloses grip apparatus comprising a housing having a hollow interior which provides a receptable to enclose a pair of grip jaws. Each of the grip jaws has a seat in its top surface adapted to receive a helical spring, mounted in the housing receptacle, to maintain the jaws in closed condition. Each of the jaws also has an inclined groove adapted to receive a lever arm of a cam mechanism that is mutually operated by a lever to release the grip jaws. The assembly comprising the grip jaws, springs, and cam is covered by a face plate which is secured by threaded fasteners onto the housing side walls. The inclined top surface of the housing must be perpendicular to the inclined wedging surface of the interior walls of the housing. Similarly, the top surface of each grip jaw is inclined and must be perpendicular to the inclined interior wall surface of the housing. That patent indicates further that removal of material from the housing weakens the housing and reduces the maximum stress to which the grip apparatus may be subjected; therefore, that patent indicates that, when slots are to be provided, the size of the housing must be increased to compensate for the loss of strength attributed to the slots. The device disclosed in that patent requires substantial disassembly of the grip apparatus to enable removal and replacement of the grip jaws. In addition, that patent does not disclose the apparatus of this invention comprising a relatively light weight housing having grip bores containing grip bars carrying grip jaw inserts that are readily removable and replaceable without substantial disassembly of the housing.
There remains a need for a grip apparatus for use with tensile stress testing devices providing a strong, relatively light weight grip apparatus that can be readily adapted for use with existing tensile stress testing devices to test high tensile strength materials without risk of failure of the grip apparatus. There remains a further need for such grip apparatus which permits easy and rapid removal and replacement of the grip jaws without removal of the grip apparatus from the test device and without substantial disassembly of the grip apparatus.